Chromium-cobalt alloy



1966 D. M. SCRUGGS CHROMIUM-COBALT ALLOY Filed March 26, 1965 CHROMIUM COBALT & w

%Cr 0R Cr SPINEL INVENTOR. DAVID M. SCRUGGS United States Patent 3,235,379 CHROMIUM-COBALT ALLOY David M. Scruggs, Sonthfield, Mich., assignor to The Bendix Corporation, Southfield, Mich., a corporation of Delaware Filed Mar. 26, 1965, Ser. No. 442,991

' 4 Claims. (Cl. 75171) The present invention is a continuation-in-part of my co-pending commonly-assigned patent application Serial Number 182,074, filed March 23, 1962, now US. Patent No. 3,175,279, granted March 30, 1965, entitled Ductile Chromium Composition which, in turn, is a continuation-in-part of my prior application Serial Number 88,- 302, filed February 10, 1961, and now abandoned.

The present invention relates to a new and improved alloy of chromium and cobalt having improved malleability and ductility, particularly at low or room temperatures compared to alloys of present commerical grades.

I have discovered that when spinels or spinel-like oxide formations in finely-divided form are distributed throughout a chromium rich cobalt alloy and said alloy is held at elevated temperatures for a period of time, the spinel acts as repositories for minute amounts of embrittling impurities such as nitrogen, oxygen, carbon, and sulphur and thereby purify the alloy, increasing the room temperature and elevated temperature malleability to a degree not heretofore obtainable, except by prohibitively expensive laboratory techniques.

Spinels are a mixture of metal monoxide and metal sesquioxide which combine according to the general formula MO-M O (or M-M O where the Ms may be the same or dis-similar metals of the appropriate valence. Monoxide and sesquioxide unite in a complex cubic crystalline structure; however, the cubic crystalline form is not critical to the practice of the present invention and certain metal oxides which combine according to the spinel formula but have tetragonal symmetry or distorted cubic symmetry rather than a perfect cubic symmetry can be used. The term spinel-like oxide used herein refers to the broader category including true spinels, inverse spinels, and those compositions corresponding to the spinel formula but having a distorted type of structure, usually tetragonal.

The true spinel structure is essentially a close packed face centered cubic array of oxygen ions with divalent cations in A; of the tetrahedral vacancy positions and trivalent cations in /z the octahedral vacancy positions. Inverse spinels exchange positions of the divalent ions with the trivalent ions. The same coordination may be held while distorted by larger divalent ions as in calcium chromate, CaCr O forming a distorted quasi-spinel structure which no longer has cubic symmetry but is tetragonal and remains functional as an electronic isomorph.

It is an object of my invention to provide a chromiumcobalt alloy having a high chromium content and which includes a small percentage of spinel or spinel-like metal oxide distributed throughout to significantly increase low and elevated temperature malleability of the alloy body even though the alloy constituents are formed of relatively impure commercial grade materials which are readily available.

Other objects and advantages of the present invention will become apparent with reference to the accompanying description and drawings.

The several test and control examples described herein were uniformly processed and tested as described below.

Alloys containing various quantities of chromium and cobalt, both with and without spinel for comparison purposes, were fabricated into pellets A3" thick by /2" diam- "ice eter by die pressing minus 325 mesh powder at 20,000 p.s.i. These pellets were then sintered in a dry hydrogen, palladium purified atmosphere in a silicon carbide resistor mufiie furnace at the temperatures listed in accompanying tables for one hour. The pellets were then ground to a flat finish on one side with emery paper and tested for hardness and malleability.

The hardness test consisted of placing the pellet in a conventional hardness test machine with a ten millimeter Brinell indenting ball in place. The load was applied in increments and the diameter of the impression made by the indenter at each increment was measured and recorded. The load was then increased to 10,000 lbs. or until the pellets cracked. The volume of metal displaced by the indenter is a function of the malleability or ductility of the material; thus, by geometrical considerations, the fourth power of the identation diameter is a deformation number which is directly related to the volume of metal displaced. This can be used to compare the amount of metal displaced at a given load which, in turn, is related to the malleability or deformability of the particular specimen. The indentation test, briefly described above, was developed by W. H. Lenz, associated with the Los Alamos Scientific Laboratories and is more fully set forth in Los Alamos Scientific Laboratory Report LAMS 2906, entitled A New Brittleness Test for Powder Metallurgy Materials, which can be obtained through the Otfice of Technical Services, United States Department of Commerce.

In Table I below, there are listed nine examples of spinel containing chromium-cobalt alloys in accordance with the teachings of the present invention fabricated and tested in the above described manner. A uniform proportion of spinel relative to chromium was maintained in Examples 1 through 9 of 6% by weight spinel relative to the chromium content.

In the above table, d represents the fourth power of the diameter of the impression made by the Brinell indenter at the load indicated in the adjacent column and is proportional to malleability.

For comparison, nine similar pellets were prepared of chromium-cobalt alloys excluding the spinel and fabricated and tested identically with Examples 1 through 9, giving the malleability or deformation data recorded in Table II below.

Table 11 Cobalt Chromium Sintering Load on Example (wt. per- (wt. percent) Temperad Iudenter cent) ture, C. (lbs) The data recorded in Tables I and II are plotted on the graph of the drawings. The graph is a plot of malleability (d versus chromium or combined chromium-spinel content of the chromium-cobalt alloy. Curve 1 is a plot of spinel containing examples 1 through 6 whereas curve 2 is a plot of the control Examples 10 through 15. At slightly greater than 40% chromium content, a sigma phase is produced which is unusable because of intermetallic formation of the constituents causes the alloy to be very brittle and hard.

The right-hand portion of FIGURE 1 is a plot above the sigma phase region wherein curve 3 is a plot of spinel containing Examples 7 through 9 and curve 4 of the control Examples 16 through 18. It will be observed that all spinel containing chromium-cobalt alloys have increased malleability compared to the controls with a general trend indicated of a greater proportionate increase with higher chromium content. This trend will continue to the 100% chromium-% cobalt point as more fully explained in my co-pending previously-referenced patent application Serial Number 182,074.

Improved malleability is obtained with spinel contents relative to chromium of less than about by Weight. Greater spinel contents produce a dispersion hardening effect and greater spinel additions become a negative factor, reducing the effectiveness of further additions. The preferred range is from 1 to 7% by weight spinel relative to chromium.

Generally, I have found that chromate and aluminate spinel-like oxides are the most effective and provide the least deleterious effect on the high temperature capability of the metal alloy. The spinels may either be added direct as spinel powder or in the case of chromate spinels, a metal oxide formed from a metal in Group II of the atomic table may be added and will form a chromate spinel during the sintering process with a somewhat impure commercial grade chromium metal as more fully described in my prior application Serial Number 182,074. From a cost standpoint, chemical inertness, high temperature capability and overall general applicability, the spinel-like oxides or mixtures thereof listed below are preferred:

(Group II oxide) Cr O Group II metal oxide-chromate spinels CaCr O Calcium chromate (distorted spinel) MgAl O Magnesium aluminate ZnAl O Zinc aluminate MgCr O Magnesium chromate NiAl O Nickel aluminate NiCr O Nickel chromate It can be appreciated that spinels are readily available, low cost, and in the percentages employed, provide minimum dilution of the alloy material. This invention provides a low cost practical means to achieve ductility in chromium-cobalt alloys not now available in wrought or cast form. The described material is readily adaptable for large scale production needs and causes a minimum effect on properties other than malleability or ductility of the base metal property.

Other metals and additives may be added to the chromium-cobalt alloy described herein for special purposes as desired and the malleability improvement described herein will be obtained as long as the spinel containing chromium-cobalt alloy is the continuous phase of the end material and such additives are in solution or form a discontinuous phase.

I claim:

1. A chromium-cobalt alloy having a continuous phase consisting of chromium, cobalt, and a spinel-like oxide dispersed therein; said spinel-like oxide comprising less than about 10% by weight relative to the chromium content; and the combined chromium and spinel-like oxide content comprising greater than 10% by weight of the continuous phase.

2. A chromium-cobalt alloy having a continuous phase consisting of chromium, cobalt, and a spinel-like oxide dispersed therein; said spinel-like oxide comprising less than about 10% by weight relative to the chromium content; and the combined chromium and spinel-like oxide content comprising from 10% to 35% by weight of the continuous phase.

3. A chromium-cobalt alloy having a continuous phase consisting of chromium, cobalt, and a spinel-like oxide dispersed therein; said spinel-like oxide comprising less than about 10% by weight relative to the chromium content; and the combined chromium and spinel-like oxide content comprising greater than by weight of the continuous phase.

4. A chromium-cobalt alloy having a continuous phase consisting of chromium, cobalt, and a spinel-like oxide dispersed therein; said spinel-like oxide selected from the groups consisting of chromate spinels, aluminate spinels, and mixtures of chromate and aluminate spinels; said spinel-like oxide comprising less than about 10% by weight relative to the chromium content; and the combined chromium and spinel-like oxide content comprising greater than 10% by weight of the continuous phase.

References Cited by the Examiner UNITED STATES PATENTS 2,671,955 3/1954 Grubel 29-1825 2,903,544 9/1959 Reichelt 29182.5

OTHER REFERENCES Scruggs: Modified Chromium for Unprotected Structures, ARS Journal, November 1961, pp. 1527-1533.

DAVID L. RECK, Primary Examiner. 

1. A CHROMIUM-COBALT ALLOY HAVING A CONTINUOUS PHASE CONSISTING OF CHROMIUM, COBALT, AND A SPINEL-LIKE OXIDE DISPERSED THEREIN; SAID SPINEL-LIKE OXIDE COMPRISING LESS THAN ABOUT 10% BY WEIGHT RELATIVE TO THE CHROMIUM CONTENT; AND THE COMBINED CHROMIUM AND SPINEL-LIKE OXIDE CONTENT COMPRISING GREATHER THAN 10% BY WEIGHT OF THE CONTINUOUS PHASE. 